If you look carefully at the ashes from different burned materials, you will see that they are not the same. Ashes of a burned log are quite different from ashes of a burned bone. And even in cases where the ashes look similar, the material that was burned to make them can usually be determined by chemical analysis. Different materials have different proportions of ash-producing elements, so determining the proportions of those elements in the ash can show what material was burned.

-- Jeff, in Minneapolis